To lower the death rate from bacteremia due to gram negative organisms we propose to treat patients with a human antiserum possessing wide protective activity. The basis for developing a single antiserum with such a wide spectrum of activity is the concept that all gram negative bacteria possess a uniform antigenic structure of the core- region lipopolysaccharide. Upon application of this concept to the treatment of endotoxemia, intravascular coagulation and overwhelming bacteremia we have been able to prevent death in experimental animals without resorting to antibiotics. Since this antiserum was effective against antibiotic-resistant organisms in immunosuppressed agranulocytic animals, it should also be of value in the treatment of hospital acquired, drug resistant infections in leukemia and other patients getting immunosuppressive drugs. By combining antiserum with conventional drug therapy for gram negative bacteremias it may be possible to lower the death rate well below the unacceptably high levels now achieved with drugs alone in these devastating infections.